1. Field of the Invention
This invention relates to a four wheel drive system for a motor vehicle and more particularly to such a system in which drive under normal conditions is to the rear wheels, power being transmitted to the front axles only when slippage at the rear axle occurs.
2. Description of the Prior Art
Usually, in a four-wheel drive vehicle, all four wheels are directly and positively driven by a transmission. While such an arrangement is effective for a vehicle traveling over difficult terrain it has several shortcomings. For example, when the vehicle is turning so that the front wheels are traveling in a wider radius than the rear wheels, because the rotational speed of all the wheels is the same with the four wheels locked to the transmission, there is a scrubbing action of the wheels on the ground. A four wheel drive vehicle traveling on a highway frequently has scrubbing action of the tire surface on the highway due to the different linear speeds of the wheels when the wheels have the same rotational speed but different diameters.
Drive mechanisms have been devised that supply drive to only one set of wheels where the four wheels of the vehicle are traveling at nearly the same speed. However, where the driving wheels begin to overrun the non-driving wheels, these wheels are engaged to the power transmission to make up for the traction lost by the wheels that normally drive the vehicle. Sometimes this is done by providing the non-driving set of wheels with an overrunning clutch and by gearing the clutch to become engaged when a predetermined speed differential between the driving and non-driving wheels occurs. This requires that the overrunning clutch be manually locked in order to obtain four wheel drive in reverse drive or when engine braking is required. Other systems use a conventional differential to allow the front and rear drive shafts to deliver power while rotating at different speeds. These systems generally require a manual lockup device to prevent excessive wheel spin when encountering surfaces on which the wheels may slip.
It is desirable in the operation of four wheel drive vehicles that the front wheels be free-rolling when they are not been driven. The front wheels should be drivably disconnected from the front axles during these non-driven periods in order to reduce drag on the engine and to avoid unnecessary wear.